imx6qdl-sabresd: Pass mmc alias

Originally, the mmc aliases node was present in imx6qdl-sabresd.dtsi.

After the sync with Linux in commit d0399a46e7 ("imx6dl/imx6qdl:
synchronise device trees with linux"), the aliases node is gone as
the upstream version does not have it.

This causes a regression in which the SD card cannot be found anymore.

Fix it by passing the alias node in the u-boot.dtsi file to
restore the original behaviour where the SD card (esdhc3) was
mapped to mmc1.

Fixes: d0399a46e7 ("imx6dl/imx6qdl: synchronise device trees with linux")
Reported-by: Carlos Rafael Giani <dv@pseudoterminal.org>
Signed-off-by: Fabio Estevam <festevam@denx.de>

arch/arm/dts/imx6qdl-sabresd-u-boot.dtsi

@@ -5,6 +5,12 @@
 
 #include "imx6qdl-u-boot.dtsi"
 
+/ {
+	aliases {
+		mmc1 = &usdhc3;
+	};
+};
+
 &usdhc3 {
 	u-boot,dm-spl;
 };